The field of the disclosure relates generally to centrifugal fans and, more specifically, to centrifugal fan impellers with variable shape fan blades for use in air moving applications.
Centrifugal fans generally use one or a combination of three basic fan blade designs: radial, forward curved, and backward curved. Generally, radial bladed fans may have fewer fan blades than forward curved and backward curved designs. In at least some radial bladed fans, the blades extend straight from the axis of rotation of the wheel hub. Typically, radial bladed fans are less efficient than forward curved and backward curved designs. At least some forward curved fans include a large number of fan blades with the blades generally having a short chord length. Forward curved blade designs have fan blades that generally curve in the direction of the wheel hub's rotation. Some forward curved fans are more efficient than radial bladed fans, but less efficient than backward curved fans. Generally, backward curved fans are more efficient than forward curved fans and radial bladed fans. Backward curved fans have fan blades that curve against the direction of the wheel hub's rotation. The blades of a backward curved fan may be flat, slightly curved, or airfoil shaped.
In general, the three basic fan blade designs result in fans with specific performance curves. Typically, a fan's performance curve is determined by the shape of the blades and cannot be altered after the fan is fabricated. As a result, the stable and unstable operating range of the fan is generally fixed. With at least some centrifugal fans, as the flow rate of air through the fan is reduced and the pressure increases, the fan blades may begin to experience stall conditions, i.e., the airflow separates form the surface of the airfoil and changes the pressure distribution to a less favorable condition. A fan with an adjustable blade design may facilitate enlarging the operating range and improving the performance curve of the fan by providing a means for the blade to change to a more favorable geometry for the specified operating point or application and reducing the tendency to stall.